Schedule II - Schedule of Balance Sheets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Condensed Financial Statements, Captions [Line Items]
Other assets	$ 129.0		$ 105.6
Total assets	6,688.9		6,378.3
Intercompany note payable	0		0
Due to subsidiaries	0		0
Accrued underwriting expense	105.0		78.5
Other liabilities	25.6		31.7
Total liabilities	5,174.8		4,915.3
Shareholders' equity	1,514.1		1,463.0		1,609.6	1,594.3
Total liabilities and shareholders' equity	6,688.9		6,378.3
Registrant [Member]
Condensed Financial Statements, Captions [Line Items]
Other investments, unrealized loss on foreign currency exchange forward contracts	(1.9)		(1.0)
Short-term investments	1.0		0.2
Investment in subsidiaries	1,621.1		1,530.6
Due from subsidiaries	5.0		0
Other assets	4.3		6.8
Total assets	1,629.5		1,536.6
Junior subordinated debentures	49.0		64.4
Intercompany note payable	55.1	[1]	0	[1]
Due to subsidiaries	0		0.3
Accrued underwriting expense	10.8		8.4
Other liabilities	0.5		0.5
Total liabilities	115.4		73.6
Shareholders' equity	1,514.1		1,463.0
Total liabilities and shareholders' equity	$ 1,629.5		$ 1,536.6

[1] For the years ended December 31, 2011 and 2010, net investment income includes intercompany dividend activity of $100.0 million and $17.5 million, respectively.